Hi Lajos,
don't know if you will read that, but your example with Model is almost working. Only a little change is to do, we have to insert a backslash before the DateTime class, because phalconphp doesn't know it:
public function getAge(){
    $from = new \DateTime($this->year.'-'.$this->month.'-'.$this->day);
    $to   = new \DateTime();
    return $from->diff($to)->y;
}
And to call the function in my template via volt, we have to do that:
 {% for birthday in user.birthday %}    
   {{  birthday.getAge() }} 
 {% endfor %}
Thx for your help
Rgds
Stefan